I am using AirReceiver in both my FireTV Stick and Bravia Android TV (not really on the latter since it is connected to my ATV 4K). I am quite positive that when I tested it on my FireTV few months ago Infuse was able to send the subtitles together with the video. So much that I though I could abandon Plex for good.
Last night I tried to Airplay a TV show, the video was playing just fine, it can even be controlled by the FireTV remote, but the subs weren't there. There is an icon for subs on the video menu, but clicking on it it looks for subtitles in the FireTV's internal "SD card" (the internal flash memory).Same thing on my Bravia. I am quite sure AirReceiver didn't receive any update (it's the same release of when I first tested it).
Ah, I have iOS 13 beta 2 on my iPhone, although I doubt it matters (at least Airplaying to Apple TV the subs are there).
Is it something that can be fixed/added on Infuse side? I do understand it's via a 3rd party app, but again I am quite sure it worked.